The present invention relates to a semiconductor device. More particularly, the present invention relates to a semiconductor device having a recess transistor.
As a semiconductor device becomes smaller and more highly integrated, a technology for reducing the size of semiconductor devices has been required. Due to the trends in miniaturization and integration of semiconductor devices, the design rule of the devices is reduced so that a channel length of a metal oxide semiconductor field effect transistor (MOSFET) is decreased.
Since the reduced channel length decreases a distance between a source region and a drain region, there is a difficulty to control of a short channel effect (SCE) where a voltage of the drain region affects voltages of the source region and the channel region. As a result, active switch elements are degraded. The narrow gap between the source region and the drain region causes a punch-through phenomenon between a source region and a drain region.
In order to prevent the punch-through phenomenon, the gate is formed by recessing the semiconductor substrate and filling it with a gate electrode, thereby increasing an effective channel length. This structure is called a recessed field effect transistor (FET) structure. The recessed FET structure improves the punch-through phenomenon, and increases a gap between the source region and the drain region.
However, a recessed gate is weak in controlling degradation of a threshold voltage Vt, thereby lowering a threshold voltage and generating a leakage current in a transistor Off state. In order to control the degradation of the threshold voltage, it is necessary to add an additional ion implanting process with impurities such as Boron. The additional ion implanting process increases an electric field, thereby degrading a refresh characteristic.
Since the semiconductor substrate is etched at a relatively deep level to form the recess gate, the swing characteristic as well as the gate controllability is degraded due to an increased body effect. Moreover, the device characteristic is degraded by the neighboring gate effect of the recess gates.